Applications of minicomputers have grown in sophistication and complexity to the point where their requirements for memory addressing capability have far exceeded the number of bits available in the computer's word length. This word length is typically 16 bits which limits the main memory of the computer to 32,768 words with infinite indirect addressing or 65,536 words if only one level of indirect addressing is allowed.
Because core memory technology had not provided economically attractive methods to provide large memory storage within a computer, past efforts were directed at mass memory devices such as magnetic discs or magnetic tape units. These mass memory devices could supply millions of bits for a small fraction of the cost of an equivalent core memory storage area. However, these devices are slow. Sometimes complex software operating systems were employed to make these devices appear to the user as an extension of the main computer memory. This technique, referred to as virtual memory, was slow and difficult to use efficiently.
Another obvious option available to the designer was to expand the word length of the computer and rearrange the architecture of the machine so that a longer word length was available for memory addressing. This was the most direct solution but also the most costly. For example, the registers, buses and the memory word length in a computer might have been expanded from 16 bits to 20 bits to increase the memory addressing capability from a maximum 65,536 to over 1,000,000 words. This option was especially unattractive when it was desirable to maintain software compatibility with previous computers because of the time and money that had been invested in building an extensive software library.
Another option that was available to the designer was bank switching. One apparatus for bank switching is described in patent application Ser. No. 360,286, entitled "Polymorphic Memory Apparatus and Method", filed by Robert J. Frankenberg on May 14, 1973. Bank switching was a cumbersome technique because of the requirement of an additional memory controller for each bank used in the system.
Also available to the designer was the use of translation tables. Prior art designs using translation tables lacked the versatility required for viable and sophisticated software operating systems. In a translation table system it is desirable to transfer words between memory areas selected by different translation tables, to execute subroutines in a memory area addressed by another translation table and to transfer program control to a program in a memory area addressed by another translation table. Furthermore, it is desirable to make some physical memory locations accessible to all users while allowing each user to also have a portion of physical memory reserved for his exclusive use.